CAD-Based Monte Carlo Radiation Transport


Monte Carlo radiation transport codes track particles as they travel through a geometric model, interacting with materials based on probability. In this project, we replace the Monte Carlo's CSG-based geometry input and modeling with a modern CAD geometry engine.  Geometric models can be constructed in modern CAD tools, with the Monte Carlo code evaluating geometry by making direct calls to the integrated CAD engine.

This work is based primarily on the CGM geometry engine and the MCNPX Monte Carlo radiation transport code.  However, since most of this functionality is generic geometry functionality, we have also successfully demonstrated this capability in the ITS and Penelope Monte Carlo codes.

Geometric Queries

Three primary types of geometric functions are called from typical Monte Carlo transport codes:

Ray Tracing


 Surface Crossing

 Surface Normal / Reflection

Since CAD-based evaluation is typically 20-100 times slower than custom geometric evaluation code in Monte Carlo codes, we also implement several special-purpose acceleration techniques to speed up this part of the calculation:
  1. Ray-tracing acceleration based on hierarchical, oriented bounding-boxes.
  2. Distance limits based on computed Monte Carlo mean free path.
  3. Non-manifold representation of multi-material models (interior surfaces shared between adjoining cells, requiring only one ray-trace calculation).

Results

We've run several test problems to verify the code and benchmark it against native MCNPX.

3 Cylinders

Cobalt Radiation Therapy Device

ARIES-CS Compact Stellerator


For more information, please email Tim Tautges (tautges _at_ engr.wisc.edu) or Paul Wilson (wilsonp _at_ engr.wisc.edu).